This invention relates to a system for supervising the integrity of a pair of transmission line conductors over which a D-C power supply is coupled to a series of d-c operated detectors, such as smoke and/or fire detectors, shunt-connected across the line. More specifically, the supervision system of the invention monitors the line's continuity, series resistance and shunt resistance, as well as the operation of an end-of-line capacitor connected across the conductor pair beyond the detectors.
Various detection systems have been developed to detect and indicate the presence of some undesired condition, such as particles of combustion, a fire, or the intrusion of a burglar. Many of these detection systems employ at least one pair of line conductors, namely a two-conductor transmission line, extending from a control panel or a central station into an area of a building to be protected. Individual d-c operated detectors, positioned at various locations in the area, are shunt-connected along the line in order to receive d-c energizing or operating voltage from a D-C power supply in the control panel, the d-c power ordinarily being produced by rectifying a-c line voltage. When an individual detector senses an undesired or alarm condition, an alarm signal is usually provided at the control panel.
It is important to supervise the line integrity, particularly for an open circuit condition which would render one or more of the detectors inoperative. This has been done in one previous system by connecting, across the end of the line, a resistance which draws more current, from the D-C power source, than the detectors under normal conditions. Integrity can be determined by monitoring the line current. Since such monitoring current flows continuously, due to the end-of-line resistor, considerable energy will be used even though an alarm condition does not arise. Moreover, it is common practice to provide a stand-by or backup battery for emergency use in case there is a power failure and a loss of the a-c line voltage. Such a battery must be of adequate size to supply the monitoring current for the duration of the emergency.
In a subsequently developed line integrity supervising system, the supervisory current required for supervision is materially reduced by replacing the end-of-line resistor with an end-of-line capacitor and by employing unfiltered, full-wave rectified a-c voltage for the d-c energizing voltage applied to the input of the two-conductor transmission line. Such an arrangement is disclosed in U.S. Pat. No. 4,191,946, issued Mar. 4, 1980, in the names of William A. Knox and Raymond J. Gonzalez. The capacitor provides sufficient filtering, when the line integrity is normal, so that the d-c voltage across the line will have a relatively small ripple component. However, if the line is open, or has an excessively high series resistance, or has an excessively low shunt resistance, or a deterioration of the end-of-line capacitor, the effect of the capacitor will be lost and a large ripple component will appear in the d-c voltage at the input end of the conductor pair. A supervisory circuit responds to the ripple component to determine when a trouble condition exists on the line, resulting in abnormal line integrity.
The approach, disclosed in the aforementioned Knox et al patent, cannot be employed, however, when the d-c energizing voltage, applied to the input of the line, has no significant ripple component and has a relatively constant instantaneous amplitude, as is the case when the d-c voltage is derived from full-wave rectified a-c line voltage which has been filtered or when the d-c voltage is derived from a stand-by battery used as an emergency power source during a power outage. This shortcoming of the Knox et al system has been overcome in the present invention by means of a unique supervision system of relatively simple and inexpensive construction. Not only can the d-c energizing voltage be ripple-free, unlike Knox et al, but considerably less supervisory current is needed in the present invention compared to that required by Knox et al.